EP2147300B1 - Method and device for non-destructive material testing of a test object using ultrasonic waves - Google Patents

Method and device for non-destructive material testing of a test object using ultrasonic waves Download PDF

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Publication number
EP2147300B1
EP2147300B1 EP08735834A EP08735834A EP2147300B1 EP 2147300 B1 EP2147300 B1 EP 2147300B1 EP 08735834 A EP08735834 A EP 08735834A EP 08735834 A EP08735834 A EP 08735834A EP 2147300 B1 EP2147300 B1 EP 2147300B1
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EP
European Patent Office
Prior art keywords
test object
test
sound
test head
material testing
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Not-in-force
Application number
EP08735834A
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German (de)
French (fr)
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EP2147300B8 (en
EP2147300A1 (en
Inventor
Rainer Boehm
Matthias Goldammer
Werner Heinrich
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Siemens AG
Original Assignee
Bundesanstalt fuer Materialforschung und Pruefung BAM
Siemens AG
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Priority to EP12001837.9A priority Critical patent/EP2469276B1/en
Publication of EP2147300A1 publication Critical patent/EP2147300A1/en
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04Analysing solids
    • G01N29/11Analysing solids by measuring attenuation of acoustic waves
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04Analysing solids
    • G01N29/06Visualisation of the interior, e.g. acoustic microscopy
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04Analysing solids
    • G01N29/06Visualisation of the interior, e.g. acoustic microscopy
    • G01N29/0654Imaging
    • G01N29/069Defect imaging, localisation and sizing using, e.g. time of flight diffraction [TOFD], synthetic aperture focusing technique [SAFT], Amplituden-Laufzeit-Ortskurven [ALOK] technique
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/44Processing the detected response signal, e.g. electronic circuits specially adapted therefor
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/44Processing the detected response signal, e.g. electronic circuits specially adapted therefor
    • G01N29/4463Signal correction, e.g. distance amplitude correction [DAC], distance gain size [DGS], noise filtering
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/04Wave modes and trajectories
    • G01N2291/044Internal reflections (echoes), e.g. on walls or defects
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/10Number of transducers
    • G01N2291/101Number of transducers one transducer

Definitions

  • the invention relates to a method for nondestructive material testing according to the preamble of claim 1. Furthermore, the invention relates to a corresponding device according to claim 16.
  • steel components are forged after casting to be subsequently turned into final shape.
  • the examination for internal material defects can already take place after forging.
  • Such metal parts are tested with ultrasound.
  • the sound waves are detected, which are reflected at interfaces in the material. With the duration of the reflected sound wave whose distance traveled can be determined.
  • further information about the material defect (s) can be obtained. From this, for example, material defects can be located. For example, the geometric orientation of the material defect can be determined in this way. From the shape of the reflected sound waves conclusions can be drawn on the nature of the material defect.
  • the volume accessible to ultrasound can be fully examined. From the collected data leaves generate an image that can be used for review.
  • the size of the material defects There are several possibilities for determining the size of the material defects. For example, during scanning, the extent of the material error can be read directly. However, this requires that the spatial resolution is smaller than the spatial extent of the material defect. The spatial resolution is limited by the wavelength used and the size of the aperture and thus by the diffraction of the sound waves.
  • the size of the material error can also be determined by the amplitude of the reflected signal. This also allows the size of such material errors to be determined that are smaller than the spatial resolution of the method. However, the amplitude of the reflected signal also depends on other parameters, such as the orientation of the material defect or the reflection properties at the interface.
  • the amplitude of the reflected signal decreases.
  • the distance to the interfering signals is too small to identify the material error from a single amplitude-transit time diagram. Conveniently, a distance of +6 dB between the measuring signal and the interfering signal is required.
  • the spatial resolution can be optimized by focusing the sound waves with the help of suitable probes.
  • the focussing can be narrowed, the wider the probe is in relation to the wavelength.
  • the focusing causes a higher sound pressure.
  • FIG. 4 shows a schematic sectional view of a test article 10 with a material defect 30.
  • a test head 16 On the outside of the test object 10 is a test head 16, which is designed as a focusing probe. Of the Probe 16 are focused sound waves 32, 34 and 36 emitted.
  • the solid line represents the wavefront of the current sound wave 32.
  • the dashed lines represent the wavefronts of the earlier sound waves 34 and the later sound waves 36.
  • the focused sound waves 32, 34 and 36 propagate along a predetermined direction with laterally limited extension.
  • the probe 16 moves along the scan 10 surface along a scan direction 38 during scanning. However, focus only occurs within the near field of the probe 16. The greater the width of the probe 16 perpendicular to the emission direction, the greater the distance of the detectable material defect 30 can be.
  • AVG method One possibility for evaluating the material errors is the evaluation of the amplitude according to the distance-gain-size method (AVG method). Based on the amplitude of the material error is assigned a replacement reflector size, which would produce a perpendicular sonicated free circular surface. If the detected signal is significantly larger than the noise signal or noise signal, the evaluation of the amplitude according to the AVG method is easily possible.
  • the reflector must be located on the acoustic axis of the sound field of the test head 16. From the dependence of the amplitude of the distance to the probe 16 corresponds to the detected amplitude of a reflector size with known geometry and orientation to the acoustic axis. If, on the other hand, the detected amplitude is smaller than the noise signal or of a comparable order of magnitude, the material error can not be identified from the amplitude-transit time diagram.
  • SAFT synthetic aperture-focus technique
  • Ahmed Yamani describes in "Three-Dimensional Imaging Using a New Synthetic Aperture Focusing Technique" (IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 44, number 4, July 1997, pages 943 to 947 ) a SAFT method using multifrequency ultrasound signals.
  • the received signal is subjected to a fury transformation with respect to the coordinates x and y via the synthetic 2D aperture, so that the pressure field is then in the form of an angle spectrum.
  • FIG. 5 For explanation of the SAFT process, a schematic sectional view of a test article 10 with a material defect 30 is shown.
  • the test head 16 On the outside of the test object 10 is the test head 16.
  • the test head has compared to FIG. 4 a relatively small diameter and is not focused. From the test head 16 spherical shell-shaped sound waves 42, 44 and 46 are emitted.
  • the wavefront of the current spherical shell-shaped sound wave 42 is represented by a solid line.
  • the dashed lines represent the wavefronts of the former spherical shell-shaped sound waves 44 and the later spherical shell-shaped sound waves 46.
  • a comparison of FIG. 4 and FIG. 5 illustrates that the wavefronts 32, 34 and 36 of the focused sound waves on the one hand and the wavefronts 42, 44 and 46 of the spherical shell-shaped sound waves on the other hand are oppositely curved.
  • the test object 10 is divided by the SAFT method of a computer into volume elements. Each volume element is considered as a reflector during the scan.
  • the reflected signal components from different positions of the probe 16, which belong to the same volume element, are recorded and added in phase with the assistance of the computer. In this way, echo signals of large amplitude are obtained only for those places with actual reflection due to constructive interference. For locations without actual reflection, the echo signals are canceled due to destructive interference.
  • the scanning and arithmetic operation simulates constructive interference an ultrasonic detector whose size corresponds to the scanned area and which is focused on a location.
  • the position of the material defect and, in the case of an extensive material defect, its size within the scope of the resolution can be determined.
  • the accuracy is approximately comparable to that in the scanned area in the aforementioned method using the focused sound waves.
  • the spatial resolution is not limited by the dimensions of the probe 16, so that a high spatial resolution is possible.
  • the SAFT method is mostly used to achieve a high spatial resolution. It is in principle a focusing method in which the resolution limit results from the wavelength and the synthetic aperture.
  • the synthetic aperture is determined by the angular range from which the material error is detected. The aperture is limited by the movement of the probe 16 and the divergence of the sound field.
  • the test object may be, for example, a rotor of a gas or steam turbine, which is used in particular for power generation. Such a rotor is exposed during operation of a high stress.
  • the speed of the rotor corresponds to the grid frequency of the respective power grid. For example, in a power grid with a mains frequency of 50 Hz, a speed of 3000 revolutions per minute is required. At such high speeds large centrifugal forces occur on the rotor. The centrifugal forces increase with the diameter of the rotor. The larger the turbine is designed, the stronger the centrifugal forces.
  • the rotors When starting the turbine, the rotors are particularly heavily loaded thermally in the tangential direction. In this phase, the rotor is initially cold and is brought by the hot combustion gases from outside to inside to operating temperature. Therefore, the number of starts for the life of the turbine has a special meaning.
  • the tangential load is greatest for the rotor in the region of its central bore. Therefore, material defects near the bore have a significant impact on the longevity of the turbines. Particularly in the coming generation of turbine wheel disks, a significant increase in the detection sensitivity for axially-radially oriented material defects is required. A sufficiently accurate determination of the axial-radial oriented material defects is not possible with the previous test methods.
  • an angle-dependent amplitude distribution is used in the sound field of the test head.
  • the core of the invention lies in a modified SAFT method in which the angle-dependent amplitude distribution in the sound field of the test head is taken into account. In this way, different sensitivities, which depend on the angle, can be considered.
  • the amplitudes of the individual reflected signals are dependent on the amplitude distribution in the sound field of the probe. It uses the spatial sound pressure distribution of the probe to determine the amplitudes of the reflected sound waves. In the conventional SAFT method, the information about the amplitude is lost.
  • a correction factor is determined which corresponds to the average sensitivity along the path through the sound field of the test head.
  • the correction factor is determined by integration over the amplitude distribution of the test head.
  • the amplitudes of the sound waves are added in phase within a predetermined angular interval about the acoustic axis.
  • a test head with a small Schalbündeldivergenz e.g. 3 ° to 5 ° at -6 dB.
  • the scanning of the surface of the test article and the variation of the insonification angles can be adapted to the geometry of the test article and the alignment of the material defects.
  • the insonification angles lie within a cone whose axis of symmetry forms the normal of the respective surface element.
  • the surface or at least the surface portion of the test object is scanned along a predetermined line. Due to the different angles of incidence, the volume of the test object can be completely detected without scanning the entire surface.
  • the surface or at least the surface portion of the test article is scanned rasterized according to a predetermined scheme.
  • This scheme can be adapted to the geometry of the test object and / or the material defect.
  • the surface or at least the surface portion of the test object can be completely scanned.
  • the insonification angles are between 0 ° and 50 °, preferably between 0 ° and 30 °.
  • the method can be provided for an at least partially rotationally symmetric test object.
  • the scanning can be particularly easily adapted to the geometry of the test object. This is especially true when the method is provided for an at least partially cylindrical test object.
  • the insonification direction preferably has a radial, tangential and / or axial component with respect to the surface of the cylindrical test object. This also particularly flat formed material errors can be detected.
  • the method is provided for material testing a metal test article, particularly for material testing of a forged component. Particularly suitable is the method for the material testing of a turbine wheel.
  • the invention relates to a device for nondestructive material testing of an at least partially massive test object, which is provided for the method described above.
  • the device comprises at least one test head for emitting ultrasonic waves and for detecting the ultrasonic waves reflected within the test object.
  • test head is pivotable, so that the insonification direction with respect to the surface normal of the surface of the test object is variable.
  • test head is pivotable with respect to the surface normal of the surface of the test object between 0 ° and 60 °, preferably between 0 ° and 30 °.
  • FIG. 1 shows a schematic sectional view of a test object 10.
  • the test object 10 is cylindrical.
  • the test object 10 has a bore 12, which is aligned concentrically to the test object 10.
  • the bore 12 and the test object 10 thus have a common rotational symmetry axis 14, which in FIG. 1 extends perpendicular to the plane of the drawing.
  • the test object 10 has an outer radius r a and an inner radius r i .
  • the inner radius r i of the test object 10 thus corresponds to the radius of the bore 12.
  • the test object 10 is a turbine disk for a gas or steam turbine.
  • test head 16 On the lateral surface of the test object 10 is a test head 16.
  • the test head 16 includes an ultrasonic transmitter and an ultrasound detector.
  • a tangential material defect 18 and a radial material defect 20 are also shown.
  • the material defects 18 and 20 each form a cavity in the test article 10.
  • the tangential material defect 18 extends with respect to the cylindrical test article 10 substantially in the tangential direction.
  • the radial material defect 20 extends substantially in the radial direction with respect to the test article 10.
  • FIG. 1 illustrates that a radial sound wave 22 is particularly strongly reflected on the tangential material defect 18 because the tangential material defect 18 is oriented substantially parallel to the surface of the test article 10. It also becomes clear that a tangential sound wave 24 is reflected particularly intensely on the radial material defect 18.
  • the sonication of the signal from the test head 16 takes place at different angles.
  • either the test head 16 itself or at least its sound-emitting component is pivotable such that the entire volume of the test object 10 is accessible by scanning the outer peripheral surface.
  • material defects 20 whose extent parallel to the surface of the test object 10 are relatively small are thereby more easily detected.
  • This is achieved in the case of the cylindrical test object 10, for example, by virtue of the fact that the insonification direction has a tangential component in addition to the radial component.
  • An insonification with a radial and an axial would be possible.
  • the insonification can also be composed of a radial, tangential and axial component.
  • the entire surface or the entire surface portion is scanned in order to detect the entire volume of the test object 10.
  • FIG. 2 is a schematic sectional view from above of the test object 10 and the test head 16 according to the embodiment in FIG. 1 shown.
  • FIG. 2 shows the bore 12, the rotational symmetry axis 14 and the radial sound wave 22.
  • the axial material defect 26 has a sufficiently large extent at least in the axial direction.
  • FIG. 2 illustrates that the radial sound wave 22 is reflected by the axial material error 26 sufficiently strong.
  • the tangential sound wave 24 would also be sufficiently strongly reflected by the axial material defect 26 at a not too large insonification angle.
  • the angle between the sound path s and the surface normal r a forms the insonification angle ⁇ or the insonification direction.
  • the sound path s and the corresponding distance vector r s of the material error 28 form a right angle ⁇ .
  • n d 2 / 4 ⁇ ⁇ ,
  • d is the width of the probe 16 and ⁇ is the wavelength of the sound wave.
  • is the wavelength of the sound wave.
  • this near-field length n can also be achieved without this width.
  • the SAFT method simulates a wide probe and thus achieves virtual focusing.
  • the amplitude of the reflected sound wave depends on the one hand on the spatial extent of the material defect 28 and on the other hand on the reflection properties at the interface of the material defect 28.
  • the first noise signal is the noise that occurs in any electronic detection system, especially in the amplifiers. This can be reduced by averaging. There is no correlation between the first noise signal and the reflected sound signal, in particular no phase correlation. The summation of the signals therefore leads to an averaging of the noise signals. With an increasing number of summands the sum of these noise signals goes to zero, if the noise signals no DC voltage component included. In practice, either no or only a small DC component occurs.
  • the second noise signal comes from the test object itself.
  • the reflections on the microstructure of the metal form a noise carpet, which correlates with the reflected sound signal.
  • the noise carpet is also a reflected sound signal. It arises from reflections in polycrystalline materials at their grain boundaries and in regions of different orientation of the crystal axes. Crystals are acoustically anisotropic, so that the wave resistance changes at the grain boundaries. In practice this affects all metal materials.
  • the individual reflections on the basis of the microstructure are not disturbing, but in broad areas of the test object 10, the noise signal comes about in this way.
  • the reflections on the structure and on the material defects can be separated by the SAFT process.
  • the Geglagerauschen shows a spatial statistics.
  • the reflections on the microstructure are phase-correlated.
  • the summation in the SAFT algorithm nevertheless leads to a relative weakening of the reflections on the microstructure, since the grain boundaries reflect less than the material errors. If an amplitude sum is achieved by a coincidentally in-phase superposition of the amplitudes of several grain boundaries, then their angle is narrowed even more. As the angular interval increases, the amplitudes due to material defects increase more than the amplitudes caused by the grain boundaries.
  • the sound field of the test head 16 is taken into account.
  • the size of the probe 16 is neglected.
  • the detected signal is produced in particular by the reflected portion of an ultrasonic pulse at sudden spatial changes in the characteristic impedance in the test object 10. These changes are interpreted as material defects if there are no design-related material boundaries or material transitions.
  • the detected signal contains only information about the amplitude and the transit time. Since the speed of sound in the material of the test object 10 is known, the distance can also be determined from the transit time. For spatial determination in the lateral direction, the spatial distribution of the sound field and the sensitivity of the test head 16 can be used.
  • the signals with the amplitude and the transit time, which are detected along the way by the test head 16, are added up with respect to the location in the test object 10 with the correct time.
  • the amplitude sum of the signals coming from a specific location in the test object increases by its amplitude with each added signal.
  • the amplitudes depend on the position of the probe 16 and thus on the relative position of the material defect 28 within the sound field.
  • the mean value of the amplitude of a material defect without directivity is proportional to its reflectance weighted by a factor k.
  • the factor k is a value for the average sensitivity along the path of the material error 18 through the sound field of the test head 16. In this way, the detected amplitude can be meaningfully evaluated.
  • the method according to the invention makes it possible to extend the application of the reflector evaluation according to the AVG method at small amplitudes by a relative reduction of the noise, as would be possible with the use of wide probes 16. This is based on the assumption that the small amplitude is due to the small size of the reflector. Therefore, the low directivity of the reflector, which is due to the diffraction, only a negligible effect on the detected amplitude.
  • the method according to the invention makes it possible to examine large test objects 10 with correspondingly large sound paths. These large sound paths cause the low amplitudes.
  • the inventive method is applicable to known classical testing techniques in which the test object is mechanically scanned and the location or the movement of the probe 16 is known to each detected amplitude-transit time diagram.
  • the evaluation of the amplitude is carried out by first the reflector is scanned by the sound field.
  • the angular dependence of the amplitude within the sound field is known.
  • M amplitudes are summed in a defined angular interval ⁇ about the acoustic axis. This results in a clear relationship between the sum of the amplitudes H Sum and the size of a reference reflector which would produce the same amplitude sum H Sum .
  • H Sum ⁇ H i ⁇ i .
  • H i are the detected amplitudes in the individual measurements and ⁇ i the angular distance to the acoustic axis.
  • ⁇ i the angular distance to the acoustic axis.
  • the correction factor k approaches a threshold corresponding to the average sensitivity in the angular interval ⁇ .
  • H AVG H Sum / m * k .
  • m the number of individual measurements
  • k a correction factor.
  • the method is suitable for minor material defects whose directivity is of minor importance.
  • FIG. 4 is a schematic sectional view of the test object 10 and a focussing probe 16 according to the prior art shown.
  • the test article 10 has a material defect 30.
  • the test head 16 On the outside of the test object 10 is the test head 16, which is designed as a focusing probe. From the probe 16 focused sound waves 32, 34 and 36 are emitted.
  • the solid line represents the wavefront of the current sound wave 32.
  • the dashed lines represent the wavefronts of the earlier sound waves 34 and the later sound waves 36.
  • the focused sound waves 32, 34 and 36 propagate along a predetermined direction with laterally limited extension.
  • the focused sound waves 18 and 20 thus do not propagate globally in the entire half space.
  • the probe 16 moves along the scan 10 surface along a scan direction 38 during scanning. However, focus only occurs within the near field of the probe 16. The greater the width of the test head 16 perpendicular to the emission direction, the greater the length of the near field and thus the penetration depth of the focused sound waves 32, 34 and 36.
  • FIG. 5 shows a schematic sectional view of the test object 10 and the test head 16 according to the SAFT method according to the prior art.
  • the test object 10 is shown with the material error 30.
  • On the outside of the test object 10 is the test head 16.
  • the test head 16 has compared to FIG. 4 a relatively small diameter and is not focused.
  • spherical shell-shaped sound waves 42, 44 and 46 are emitted.
  • the wavefront of the current spherical shell-shaped sound wave 42 is represented by a solid line.
  • the dashed lines represent the wavefronts of the former spherical shell-shaped Sound waves 44 and the later spherical shell-shaped sound waves 46.
  • FIG. 4 and FIG. 5 illustrates that the wavefronts 32, 34 and 36 on the one hand and 42, 44 and 46 on the other hand are oppositely curved.
  • the test object 10 is subdivided by a computer into volume elements in this SAFT method. Each volume element is considered as a reflector during the scan.
  • the reflected signal components from different positions of the probe 16, which belong to the same volume element, are recorded and added in phase with the assistance of the computer. In this way, echo signals of large amplitude are obtained only for those places with actual reflection due to constructive interference.
  • the echo signals are canceled due to destructive interference.
  • the scanning and arithmetic operation simulates constructive interference an ultrasonic detector whose size corresponds to the scanned area.
  • the insonification angle is always 0 ° and the entire surface of the test object 10 is scanned.
  • the insonification angle ⁇ can be varied.
  • the method according to the invention is not limited to the cylindrical test object 10, such as wheel disks or shafts.
  • the insonification direction can be composed of suitable base vectors, which are adapted to the geometric shape of the test object 10.
  • the method according to the invention leads to a substantial improvement in the detectability of small material defects and those which are located deep inside the test object 10.

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Description

Die Erfindung betrifft ein Verfahren zur zerstörungsfreien Materialprüfung gemäß dem Oberbegriff des Patentanspruches 1. Weiterhin betrifft die Erfindung eine entsprechende Vorrichtung gemäß dem Patentanspruch 16.The invention relates to a method for nondestructive material testing according to the preamble of claim 1. Furthermore, the invention relates to a corresponding device according to claim 16.

Bei zahlreichen massiven und teilweise massiven Produkten und auch Zwischenprodukten muss deren innere Struktur nach Materialfehlern untersucht werden. Dazu sind zerstörungsfreie Prüfverfahren erforderlich, die Informationen über die innere, nicht-einsehbare Struktur bereitstellen. Dies ist insbesondere bei mechanisch stark beanspruchten Bauteilen notwendig.In the case of many massive and sometimes massive products as well as intermediates, their internal structure must be investigated for material defects. This requires non-destructive testing methods that provide information about the internal, non-observable structure. This is particularly necessary for mechanically stressed components.

Beispielsweise werden Bauteile aus Stahl nach dem Gießen geschmiedet, um anschließend durch Drehen in die endgültige Form gebracht zu werden. Dabei kann die Prüfung auf innere Materialfehler bereits nach dem Schmieden erfolgen.For example, steel components are forged after casting to be subsequently turned into final shape. In this case, the examination for internal material defects can already take place after forging.

Üblicherweise werden solche Metallteile mit Ultraschall geprüft. Dabei werden die Schallwellen erfasst, die an Grenzflächen im Material reflektiert werden. Mit der Laufzeit der reflektierten Schallwelle kann deren zurückgelegte Weglänge bestimmt werden. Durch eine Einschallung aus verschiedenen Richtungen lassen sich weitere Informationen über den oder die Materialfehler gewinnen. Daraus lassen sich beispielsweise Materialfehler orten. Beispielsweise kann die geometrische Ausrichtung des Materialfehlers auf diese Weise bestimmt werden. Aus der Form der reflektierten Schallwellen lassen sich Rückschlüsse auf die Art des Materialfehlers ziehen.Usually, such metal parts are tested with ultrasound. The sound waves are detected, which are reflected at interfaces in the material. With the duration of the reflected sound wave whose distance traveled can be determined. By sounding in from different directions, further information about the material defect (s) can be obtained. From this, for example, material defects can be located. For example, the geometric orientation of the material defect can be determined in this way. From the shape of the reflected sound waves conclusions can be drawn on the nature of the material defect.

Durch ein Abtasten der Oberfläche des Prüfgegenstands mit einem Ultraschalldetektor und ein Aufzeichnen der erfassten Daten kann das dem Ultraschall zugängliche Volumen vollständig untersucht werden. Aus den erfassten Daten lässt sich eine Abbildung generieren, die zur Begutachtung verwendet werden kann.By scanning the surface of the test object with an ultrasound detector and recording the acquired data, the volume accessible to ultrasound can be fully examined. From the collected data leaves generate an image that can be used for review.

Zur Bestimmung der Größe der Materialfehler gibt es mehrere Möglichkeiten. Beispielsweise kann beim Abtasten die Ausdehnung des Materialfehlers direkt abgelesen werden. Dazu ist jedoch erforderlich, dass die Ortsauflösung kleiner als die räumliche Ausdehnung des Materialfehlers ist. Die Ortsauflösung ist durch die verwendete Wellenlänge und die Größe der Apertur und damit durch die Beugung der Schallwellen begrenzt.There are several possibilities for determining the size of the material defects. For example, during scanning, the extent of the material error can be read directly. However, this requires that the spatial resolution is smaller than the spatial extent of the material defect. The spatial resolution is limited by the wavelength used and the size of the aperture and thus by the diffraction of the sound waves.

Die Größe des Materialfehlers kann auch mit der Amplitude des reflektierten Signals bestimmt werden. Damit lässt sich auch die Größe solcher Materialfehler bestimmen, die kleiner als die Ortsauflösung des Verfahrens sind. Die Amplitude des reflektierten Signals hängt jedoch auch von weiteren Parametern ab, beispielsweise von der Orientierung des Materialfehlers oder den Reflexionseigenschaften an der Grenzfläche.The size of the material error can also be determined by the amplitude of the reflected signal. This also allows the size of such material errors to be determined that are smaller than the spatial resolution of the method. However, the amplitude of the reflected signal also depends on other parameters, such as the orientation of the material defect or the reflection properties at the interface.

Bei abnehmender Größe des Materialfehlers nimmt die Amplitude des reflektierten Signals ab. Dabei wird der Abstand zu den Störsignalen zu gering, um aus einem einzigen Amplituden-Laufzeit-Diagramm den Materialfehler zu identifizieren. Zweckmäßigerweise ist ein Abstand von +6 dB zwischen dem Messsignal und dem Störsignal erforderlich.As the size of the material defect decreases, the amplitude of the reflected signal decreases. The distance to the interfering signals is too small to identify the material error from a single amplitude-transit time diagram. Conveniently, a distance of +6 dB between the measuring signal and the interfering signal is required.

Die Ortsauflösung lässt sich durch eine Fokussierung der Schallwellen mit Hilfe geeigneter Prüfköpfe optimieren. Dabei kann die Fokussierung umso schmäler werden, je breiter der Prüfkopf im Verhältnis zur Wellenlänge ist. Die Fokussierung bewirkt einen höheren Schalldruck.The spatial resolution can be optimized by focusing the sound waves with the help of suitable probes. In this case, the focussing can be narrowed, the wider the probe is in relation to the wavelength. The focusing causes a higher sound pressure.

FIG. 4 zeigt eine schematische Schnittansicht eines Prüfgegenstands 10 mit einem Materialfehler 30. An der Außenseite des Prüfgegenstands 10 befindet sich ein Prüfkopf 16, der als fokussierender Prüfkopf ausgebildet ist. Von dem Prüfkopf 16 werden fokussierte Schallwellen 32, 34 und 36 abgestrahlt. Dabei stellt die durchgezogene Linie die Wellenfront der aktuellen Schallwelle 32 dar. Die gestrichelten Linien stellen die Wellenfronten der früheren Schallwellen 34 und der späteren Schallwellen 36 dar. Die fokussierten Schallwellen 32, 34 und 36 breiten sich entlang einer vorbestimmten Richtung mit seitlich begrenzter Ausdehnung aus. FIG. 4 shows a schematic sectional view of a test article 10 with a material defect 30. On the outside of the test object 10 is a test head 16, which is designed as a focusing probe. Of the Probe 16 are focused sound waves 32, 34 and 36 emitted. The solid line represents the wavefront of the current sound wave 32. The dashed lines represent the wavefronts of the earlier sound waves 34 and the later sound waves 36. The focused sound waves 32, 34 and 36 propagate along a predetermined direction with laterally limited extension.

Der Prüfkopf 16 bewegt sich während des Abtastens auf der Oberfläche des Prüfgegenstands 10 entlang einer Abtastrichtung 38. Die Fokussierung tritt jedoch nur innerhalb des Nahfelds des Prüfkopfs 16 auf. Je größer die Breite des Prüfkopfs 16 senkrecht zur Abstrahlrichtung ist, desto größer kann die Entfernung des erfassbaren Materialfehlers 30 sein.The probe 16 moves along the scan 10 surface along a scan direction 38 during scanning. However, focus only occurs within the near field of the probe 16. The greater the width of the probe 16 perpendicular to the emission direction, the greater the distance of the detectable material defect 30 can be.

Eine Möglichkeit zur Bewertung der Materialfehler ist die Auswertung der Amplitude nach dem Abstand-Verstärkung-Größe-Verfahren (AVG-Verfahren). Ausgehend von der Amplitude wird dem Materialfehler eine Ersatzreflektorgröße zugeordnet, die eine senkrecht beschallte freie kreisrunde Oberfläche erzeugen würde. Wenn das erfasste Signal deutlich größer ist als das Störsignal oder Rauschsignal, ist die Auswertung der Amplitude nach der AVG-Methode problemlos möglich. Dabei muss sich der Reflektor auf der akustischen Achse des Schallfeldes des Prüfkopfes 16 befinden. Aus der Abhängigkeit der Amplitude vom Abstand zum Prüfkopf 16 entspricht die erfasste Amplitude einer Reflektorgröße mit bekannter Geometrie und Orientierung zur akustischen Achse. Ist dagegen die erfasste Amplitude kleiner als das Rauschsignal oder in einer vergleichbaren Größenordnung, kann der Materialfehler aus dem Amplituden-Laufzeit-Diagramm nicht identifiziert werden.One possibility for evaluating the material errors is the evaluation of the amplitude according to the distance-gain-size method (AVG method). Based on the amplitude of the material error is assigned a replacement reflector size, which would produce a perpendicular sonicated free circular surface. If the detected signal is significantly larger than the noise signal or noise signal, the evaluation of the amplitude according to the AVG method is easily possible. The reflector must be located on the acoustic axis of the sound field of the test head 16. From the dependence of the amplitude of the distance to the probe 16 corresponds to the detected amplitude of a reflector size with known geometry and orientation to the acoustic axis. If, on the other hand, the detected amplitude is smaller than the noise signal or of a comparable order of magnitude, the material error can not be identified from the amplitude-transit time diagram.

Eine andere Methode zur Verbesserung der Ortsauflösung ist die "Synthetische Apertur-Fokus-Technik" (SAFT), bei der ein kleiner nicht fokussierender Prüfkopf verwendet wird. Dabei wird mit einer zweidimensionalen mechanischen Abtastung des Prüfgegenstands eine dreidimensionale Abbildung des Prüfgegenstands berechnet.Another way to improve spatial resolution is the "synthetic aperture-focus technique" (SAFT), which uses a small non-focusing probe. This is done with a two-dimensional mechanical scanning of Test object calculated a three-dimensional image of the test object.

Martin Spies und Winfried Jager beschreiben in "Synthetic aperture focusing for defect reconstruction in anisotropic media" (Ultrasonics 41 (2003), Seiten 125 bis 131 ) ein SAFT-Verfahren für anisotrope Medien. In solchen Medien hängt die Gruppengeschwindigkeit des Ultraschallsignals von der Ausbreitungsrichtung ab. Als Folge davon erfolgt eine Schrägstellung des Signals gegenüber der eigentlichen Ausbreitungsrichtung. Um die Anisotopie berücksichtigen zu können, wird eine Versetzungsamplitude (displacement amplitude) für das Wellenfeld an einer Position Rj ermittelt, die dann zur eigentlichen Berechnung des Bildes anhand des SAFT-Algorithmus herangezogen wird. Martin Spies and Winfried Jager describe in "Synthetic aperture focusing for defect reconstruction in anisotropic media" (Ultrasonics 41 (2003), pages 125 to 131 ) a SAFT method for anisotropic media. In such media, the group velocity of the ultrasonic signal depends on the propagation direction. As a consequence, the signal is skewed relative to the actual propagation direction. In order to take the anisotropy into account, a displacement amplitude for the wave field is determined at a position R j , which is then used to actually calculate the image using the SAFT algorithm.

Ahmed Yamani beschreibt in "Three-Dimensional Imaging Using a New Synthetic Aperture Focusing Technique" (IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 44, number 4, July 1997, pages 943 to 947 ) ein SAFT-Verfahren unter Verwendung multifrequenter Ultraschallsignale. In diesem Verfahren wird das empfangene Signal über die synthetische 2D-Apertur einer Furie-Transformation bezüglich der Koordinaten x und y unterzogen, so dass das Druckfeld dann in Form eines Winkelspektrums vorliegt. Ahmed Yamani describes in "Three-Dimensional Imaging Using a New Synthetic Aperture Focusing Technique" (IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 44, number 4, July 1997, pages 943 to 947 ) a SAFT method using multifrequency ultrasound signals. In this method, the received signal is subjected to a fury transformation with respect to the coordinates x and y via the synthetic 2D aperture, so that the pressure field is then in the form of an angle spectrum.

In FIG. 5 ist zur Erläuterung des SAFT-Verfahrens eine schematische Schnittansicht eines Prüfgegenstands 10 mit einem Materialfehler 30 dargestellt. An der Außenseite des Prüfgegenstands 10 befindet sich der Prüfkopf 16. Der Prüfkopf hat im Vergleich zu FIG. 4 einen relativ kleinen Durchmesser und ist nicht fokussierend ausgebildet. Von dem Prüfkopf 16 werden kugelschalenförmige Schallwellen 42, 44 und 46 abgestrahlt. Die Wellenfront der aktuellen kugelschalenförmigen Schallwelle 42 ist durch eine durchgezogene Linie dargestellt. Die gestrichelten Linien stellen die Wellenfronten der früheren kugelschalenförmigen Schallwellen 44 sowie der späteren kugelschalenförmigen Schallwellen 46 dar. Ein Vergleich von FIG. 4 und FIG. 5 verdeutlicht, dass die Wellenfronten 32, 34 und 36 der fokussierten Schallwellen einerseits und die Wellenfronten 42, 44 und 46 der kugelschalenförmigen Schallwellen andererseits entgegengesetzt gekrümmt sind.In FIG. 5 For explanation of the SAFT process, a schematic sectional view of a test article 10 with a material defect 30 is shown. On the outside of the test object 10 is the test head 16. The test head has compared to FIG. 4 a relatively small diameter and is not focused. From the test head 16 spherical shell-shaped sound waves 42, 44 and 46 are emitted. The wavefront of the current spherical shell-shaped sound wave 42 is represented by a solid line. The dashed lines represent the wavefronts of the former spherical shell-shaped sound waves 44 and the later spherical shell-shaped sound waves 46. A comparison of FIG. 4 and FIG. 5 illustrates that the wavefronts 32, 34 and 36 of the focused sound waves on the one hand and the wavefronts 42, 44 and 46 of the spherical shell-shaped sound waves on the other hand are oppositely curved.

Der Prüfgegenstand 10 wird beim SAFT-Verfahren von einem Rechner in Volumenelemente unterteilt. Jedes Volumenelement wird während der Abtastung nacheinander als Reflektor betrachtet. Die reflektierten Signalanteile von verschiedenen Positionen des Prüfkopfs 16, die zu demselben Volumenelement gehören, werden aufgezeichnet und mit Unterstützung des Rechners phasenrichtig aufaddiert. Auf diese Weise werden nur für solche Orte mit tatsächlicher Reflexion aufgrund der konstruktiven Interferenz Echosignale mit großer Amplitude erhalten. Für Orte ohne tatsächliche Reflexion werden aufgrund der destruktiven Interferenz die Echosignale ausgelöscht. Der Abtast- und Rechenvorgang simuliert bei konstruktiver Interferenz einen Ultraschalldetektor, dessen Größe der abgetasteten Fläche entspricht und der auf einen Ort fokussiert ist.The test object 10 is divided by the SAFT method of a computer into volume elements. Each volume element is considered as a reflector during the scan. The reflected signal components from different positions of the probe 16, which belong to the same volume element, are recorded and added in phase with the assistance of the computer. In this way, echo signals of large amplitude are obtained only for those places with actual reflection due to constructive interference. For locations without actual reflection, the echo signals are canceled due to destructive interference. The scanning and arithmetic operation simulates constructive interference an ultrasonic detector whose size corresponds to the scanned area and which is focused on a location.

Daraus lässt sich die Position des Materialfehlers und bei einem ausgedehnten Materialfehler auch dessen Größe im Rahmen der Auflösung bestimmen. Die Genauigkeit ist etwa vergleichbar mit der in dem abgetasteten Bereich bei dem vorgenannten Verfahren, das die fokussierten Schallwellen verwendet. Bei dem SAFT-Verfahren ist die Ortsauflösung nicht durch die Abmessungen des Prüfkopfs 16 begrenzt, so dass eine hohe Ortsauflösung möglich ist.From this, the position of the material defect and, in the case of an extensive material defect, its size within the scope of the resolution can be determined. The accuracy is approximately comparable to that in the scanned area in the aforementioned method using the focused sound waves. In the SAFT method, the spatial resolution is not limited by the dimensions of the probe 16, so that a high spatial resolution is possible.

Beim SAFT-Verfahren werden in jedem Bildpunkt im Fehlererwartungsbereich alle in Frage kommenden reflektierten Signalanteile mit einer Zeitverschiebung addiert, die die Signalanteile dann hätten, wenn der Bildpunkt die Quelle einer reflektierten Welle wäre. Die Zeitverschiebung, die der Phasenlage entspricht, ergibt sich aus den geometrischen Beziehungen zwischen dem Prüfkopf 16 und dem Bildpunkt, insbesondere aus dem Abstand zwischen dem Prüfkopf 16 und dem Bildpunkt. Wenn der Bildpunkt nun tatsächlich die Quelle einer reflektierten Welle ist, dann nimmt die Amplitude an dieser Stelle mit der Anzahl der verschiedenen Positionen des Prüfkopfs 16 zu, von denen aus der Materialfehler erfasst wurde. Für alle anderen Bildpunkte stimmen die Phasen nicht überein, so dass die Summe im Idealfall gegen Null geht, zumindest aber sehr klein ist.In the SAFT method, in each pixel in the error expectancy range, all possible reflected signal components are added with a time shift which the signal components would have if the pixel were the source of a reflected wave. The time shift, which corresponds to the phase position, results from the geometric relationships between the probe 16 and the pixel, in particular from the distance between the probe 16 and the pixel. Now, if the pixel is actually the source of a reflected wave, then the amplitude at that location increases with the number of different positions of the probe 16 from which the material error was detected. For all other pixels, the phases do not match, so that ideally the sum approaches zero, or at least is very small.

Das SAFT-Verfahren wird meist verwendet, um eine hohe Ortsauflösung zu erreichen. Es handelt sich im Prinzip um ein Fokussierungsverfahren, bei dem sich die Auflösungsgrenze durch die Wellenlänge und die synthetische Apertur ergibt. Die synthetische Apertur wird von dem Winkelbereich bestimmt, aus dem der Materialfehler erfasst wird. Die Apertur wird durch die Bewegung des Prüfkopfes 16 und die Divergenz des Schallfeldes begrenzt.The SAFT method is mostly used to achieve a high spatial resolution. It is in principle a focusing method in which the resolution limit results from the wavelength and the synthetic aperture. The synthetic aperture is determined by the angular range from which the material error is detected. The aperture is limited by the movement of the probe 16 and the divergence of the sound field.

Der Prüfgegenstand kann beispielsweise ein Rotor einer Gas- oder Dampfturbine sein, der insbesondere für die Stromerzeugung verwendet wird. Ein solcher Rotor wird im Betrieb einer hohen Beanspruchung ausgesetzt. Die Drehzahl des Rotors entspricht der Netzfrequenz des jeweiligen Stromnetzes. Beispielsweise ist bei einem Stromnetz mit einer Netzfrequenz von 50 Hz eine Drehzahl von 3000 Umdrehungen pro Minute erforderlich. Bei derart hohen Drehzahlen treten große Fliehkräfte am Rotor auf. Die Fliehkräfte nehmen mit dem Durchmesser des Rotors zu. Je größer die Turbine ausgelegt ist, desto stärker sind auch die Fliehkräfte.The test object may be, for example, a rotor of a gas or steam turbine, which is used in particular for power generation. Such a rotor is exposed during operation of a high stress. The speed of the rotor corresponds to the grid frequency of the respective power grid. For example, in a power grid with a mains frequency of 50 Hz, a speed of 3000 revolutions per minute is required. At such high speeds large centrifugal forces occur on the rotor. The centrifugal forces increase with the diameter of the rotor. The larger the turbine is designed, the stronger the centrifugal forces.

Beim Start der Turbine werden die Rotoren insbesondere thermisch in tangentialer Richtung stark belastet. In dieser Phase ist der Rotor zunächst kalt und wird durch die heißen Verbrennungsgase von Außen nach Innen auf Betriebstemperatur gebracht. Daher hat für die Lebensdauer der Turbine die Anzahl der Starts eine besondere Bedeutung. Die tangentiale Belastung ist für den Rotor im Bereich seiner zentralen Bohrung am größten. Daher haben Materialfehler in der Nähe der Bohrung einen entscheidenden Einfluss auf die Langlebigkeit der Turbinen. Insbesondere bei der kommenden Generation von Turbinenradscheiben ist eine deutliche Erhöhung der Nachweisempfindlichkeit für axial-radial orientierte Materialfehler erforderlich. Eine hinreichend genaue Bestimmung der axial-radial orientierten Materialfehler ist mit den bisherigen Prüfverfahren nicht möglich.When starting the turbine, the rotors are particularly heavily loaded thermally in the tangential direction. In this phase, the rotor is initially cold and is brought by the hot combustion gases from outside to inside to operating temperature. Therefore, the number of starts for the life of the turbine has a special meaning. The tangential load is greatest for the rotor in the region of its central bore. Therefore, material defects near the bore have a significant impact on the longevity of the turbines. Particularly in the coming generation of turbine wheel disks, a significant increase in the detection sensitivity for axially-radially oriented material defects is required. A sufficiently accurate determination of the axial-radial oriented material defects is not possible with the previous test methods.

Aufgrund der höheren Leistungen der neueren Gas- oder Dampfturbinen steigen die Anforderungen, dass der Rotor frei von Materialfehlern ist. Auch die Größe der Rotoren nimmt zu, was bei der Materialprüfung längere Ultraschallwege zur Folge hat. Durch die größere Weglänge des Ultraschalls nimmt im inneren Bereich des Rotors die Mindestgröße der erfassbaren Materialfehler zu. Es besteht somit ein Bedarf an einem Verfahren, das auch eine Materialfehlerbestimmung bei großen Bauteilen ermöglicht.Due to the higher performance of the newer gas or steam turbines, the requirements increase that the rotor is free from material defects. The size of the rotors also increases, which results in longer ultrasonic paths during material testing. Due to the greater path length of the ultrasound, the minimum size of the detectable material defects increases in the inner region of the rotor. Thus, there is a need for a method that also enables material defect determination for large components.

Es ist Aufgabe der Erfindung, ein verbessertes Verfahren zum Auffinden und/oder Identifizieren von Materialfehlern in einem Prüfgegenstand bereit zu stellen, das auch bei relativ großen Prüfgegenständen eine Materialfehlerbestimmung mit hinreichender Genauigkeit ermöglicht.It is an object of the invention to provide an improved method for finding and / or identifying material defects in a test article, which is also at relative large inspection items a material defect determination with sufficient accuracy allows.

Diese Aufgabe wird durch den Gegenstand gemäß Patentanspruch 1 gelöst.This object is achieved by the subject matter of claim 1.

Gemäß der Erfindung ist vorgesehen, dass eine winkelabhängige Amplitudenverteilung im Schallfeld des Prüfkopfes verwendet wird.According to the invention, it is provided that an angle-dependent amplitude distribution is used in the sound field of the test head.

Der Kern der Erfindung liegt in einem modifizierten SAFT-Verfahren, bei dem die winkelabhängige Amplitudenverteilung im Schallfeld des Prüfkopfes berücksichtigt wird. Auf diese Weise lassen sich unterschiedliche Empfindlichkeiten, die vom Winkel abhängen, berücksichtigen. Die Amplituden der einzelnen reflektierten Signale sind abhängig von der Amplitudenverteilung im Schallfeld des Prüfkopfes. Es wird die räumliche Schalldruckverteilung des Prüfkopfes genutzt, um auch die Amplituden der reflektierten Schallwellen zu bestimmen. Beim herkömmlichen SAFT-Verfahren geht die Information über die Amplitude verloren.The core of the invention lies in a modified SAFT method in which the angle-dependent amplitude distribution in the sound field of the test head is taken into account. In this way, different sensitivities, which depend on the angle, can be considered. The amplitudes of the individual reflected signals are dependent on the amplitude distribution in the sound field of the probe. It uses the spatial sound pressure distribution of the probe to determine the amplitudes of the reflected sound waves. In the conventional SAFT method, the information about the amplitude is lost.

Beispielsweise wird aus der winkelabhängigen Amplitudenverteilung ein Korrekturfaktor bestimmt, der der mittleren Empfindlichkeit entlang des Weges durch das Schallfeld des Prüfkopfes entspricht. Der Korrekturfaktor wird durch Integration über die Amplitudenverteilung des Prüfkopfes bestimmt.For example, from the angle-dependent amplitude distribution, a correction factor is determined which corresponds to the average sensitivity along the path through the sound field of the test head. The correction factor is determined by integration over the amplitude distribution of the test head.

Vorzugsweise werden die Amplituden der Schallwellen innerhalb eines vorbestimmten Winkelintervalls um die akustische Achse phasenrichtig aufaddiert. Dabei kann auch ein Prüfkopf mit einer kleinen Schalbündeldivergenz, z.B. 3° bis 5° bei -6 dB, verwendet werden.Preferably, the amplitudes of the sound waves are added in phase within a predetermined angular interval about the acoustic axis. In this case, a test head with a small Schalbündeldivergenz, e.g. 3 ° to 5 ° at -6 dB.

Weiterhin kann das Beaufschlagen des Prüfgegenstands mit Ultraschall unter verschiedenen Einschallwinkeln bezüglich des Flächenelements an der Oberfläche des Prüfgegenstands erfolgen. Da Materialfehler oftmals eine bevorzugte Ausdehnungsrichtung haben, können gemäß der Erfindung das Abtasten der Oberfläche des Prüfgegenstands und die Variation der Einschallwinkel an die Geometrie des Prüfgegenstands und die Ausrichtung der Materialfehler angepasst werden.Furthermore, ultrasonically applying the test specimen at different angles of incidence with respect to the surface element at the surface of the test object respectively. Since flaws often have a preferred direction of elongation, according to the invention the scanning of the surface of the test article and the variation of the insonification angles can be adapted to the geometry of the test article and the alignment of the material defects.

Beispielsweise liegen die Einschallwinkel innerhalb eines Kegels, dessen Symmetrieachse die Normale des jeweiligen Flächenelements bildet.For example, the insonification angles lie within a cone whose axis of symmetry forms the normal of the respective surface element.

Bei einer speziellen Ausführungsform kann vorgesehen sein, dass die Oberfläche oder zumindest der Oberflächenabschnitt des Prüfgegenstands entlang einer vorbestimmten Linie abgetastet wird. Durch die verschiedenen Einschallwinkel kann das Volumen des Prüfgegenstands vollständig erfasst werden, ohne die gesamte Oberfläche abzutasten.In a specific embodiment it can be provided that the surface or at least the surface portion of the test object is scanned along a predetermined line. Due to the different angles of incidence, the volume of the test object can be completely detected without scanning the entire surface.

Vorzugsweise wird die Oberfläche oder zumindest der Oberflächenabschnitt des Prüfgegenstands gerastert nach einem vorbestimmten Schema abgetastet. Dieses Schema kann an die Geometrie des Prüfgegenstands und/oder des Materialfehlers angepasst werden.Preferably, the surface or at least the surface portion of the test article is scanned rasterized according to a predetermined scheme. This scheme can be adapted to the geometry of the test object and / or the material defect.

Weiterhin kann auch die Oberfläche oder zumindest der Oberflächenabschnitt des Prüfgegenstands vollständig abgetastet werden.Furthermore, the surface or at least the surface portion of the test object can be completely scanned.

Beispielsweise betragen die Einschallwinkel zwischen 0° und 50°, vorzugsweise zwischen 0° und 30°.For example, the insonification angles are between 0 ° and 50 °, preferably between 0 ° and 30 °.

Insbesondere kann das Verfahren für einen zumindest abschnittsweise rotationssymmetrischen Prüfgegenstand vorgesehen sein. Dabei lässt sich das Abtasten besonders einfach an die Geometrie des Prüfgegenstands anpassen. Dies trifft insbesondere dann zu, wenn das Verfahren für einen zumindest abschnittsweise zylindrischen Prüfgegenstand vorgesehen ist.In particular, the method can be provided for an at least partially rotationally symmetric test object. In this case, the scanning can be particularly easily adapted to the geometry of the test object. This is especially true when the method is provided for an at least partially cylindrical test object.

Dabei weist die Einschallrichtung vorzugsweise eine radiale, tangentiale und/oder axiale Komponente bezüglich der Oberfläche des zylindrischen Prüfgegenstands auf. Damit können auch besonders flach ausgebildete Materialfehler erkannt werden.In this case, the insonification direction preferably has a radial, tangential and / or axial component with respect to the surface of the cylindrical test object. This also particularly flat formed material errors can be detected.

Bei der bevorzugten Ausführungsform ist das Verfahren für die Materialprüfung eines Prüfgegenstands aus Metall vorgesehen, insbesondere für die Materialprüfung eines geschmiedeten Bauteils. Besonders geeignet ist das Verfahren für die Materialprüfung eines Turbinenrades.In the preferred embodiment, the method is provided for material testing a metal test article, particularly for material testing of a forged component. Particularly suitable is the method for the material testing of a turbine wheel.

Weiterhin betrifft die Erfindung eine Vorrichtung zur zerstörungsfreien Materialprüfung eines zumindest abschnittsweise massiven Prüfgegenstands, die für das oben beschriebene Verfahren vorgesehen ist.Furthermore, the invention relates to a device for nondestructive material testing of an at least partially massive test object, which is provided for the method described above.

Vorzugsweise umfasst die Vorrichtung wenigstens einen Prüfkopf zum Aussenden von Ultraschallwellen und zum Erfassen der innerhalb des Prüfgegenstands reflektierten Ultraschallwellen.Preferably, the device comprises at least one test head for emitting ultrasonic waves and for detecting the ultrasonic waves reflected within the test object.

Insbesondere ist der Prüfkopf schwenkbar, so dass die Einschallrichtung bezüglich der Flächennormalen der Oberfläche des Prüfgegenstands variierbar ist.In particular, the test head is pivotable, so that the insonification direction with respect to the surface normal of the surface of the test object is variable.

Schließlich ist der Prüfkopf bezüglich der Flächennormalen der Oberfläche des Prüfgegenstands zwischen 0° und 60°, vorzugsweise zwischen 0° und 30° schwenkbar.Finally, the test head is pivotable with respect to the surface normal of the surface of the test object between 0 ° and 60 °, preferably between 0 ° and 30 °.

Weitere Merkmale, Vorteile und besondere Ausführungsformen der Erfindung sind Gegenstand der Unteransprüche.Further features, advantages and particular embodiments of the invention are the subject of the dependent claims.

Nachstehend wird das Verfahren gemäß der Erfindung in der Figurenbeschreibung anhand bevorzugter Ausführungsformen und unter Bezugnahme auf die beigefügten Zeichnungen näher erläutert. Es zeigen:

FIG. 1
eine schematische seitliche Schnittansicht eines Prüfgegenstands und eines Prüfkopfes gemäß einer bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens,
FIG. 2
eine schematische Schnittansicht von oben auf den Prüfgegenstand und den Prüfkopf gemäß der bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens,
FIG. 3
eine schematische Skizze über die geometrischen Beziehungen des Prüfgegenstands, des Prüfkopfs und eines Materialsfehlers bei der bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens,
FIG. 4
eine schematische Schnittansicht auf den Prüfgegenstand und einen fokussierenden Prüfkopf gemäß dem Stand der Technik, und
FIG. 5
eine schematische Schnittansicht auf den Prüfgegenstand und einen Prüfkopf nach dem SAFT-Verfahren gemäß dem Stand der Technik.
Hereinafter, the method according to the invention in the figure description with reference to preferred embodiments and with reference to the accompanying drawings will be explained in more detail. Show it:
FIG. 1
3 is a schematic sectional side view of a test object and a test head according to a preferred embodiment of the method according to the invention,
FIG. 2
1 is a schematic sectional view from above of the test object and the test head according to the preferred embodiment of the method according to the invention,
FIG. 3
a schematic sketch of the geometric relationships of the test object, the test head and a material error in the preferred embodiment of the method according to the invention,
FIG. 4
a schematic sectional view of the test object and a focusing probe according to the prior art, and
FIG. 5
a schematic sectional view of the test object and a test head according to the SAFT method according to the prior art.

FIG. 1 zeigt eine schematische Schnittansicht eines Prüfgegenstands 10. Der Prüfgegenstand 10 ist zylinderförmig ausgebildet. Der Prüfgegenstand 10 weist eine Bohrung 12 auf, die konzentrisch zum Prüfgegenstand 10 ausgerichtet ist. Die Bohrung 12 und der Prüfgegenstand 10 weisen somit eine gemeinsame Rotationssymmetrieachse 14 auf, die in FIG. 1 senkrecht zur Zeichnungsebene erstreckt. Der Prüfgegenstand 10 weist einen Außenradius ra und einen Innenradius ri auf. Der Innenradius ri des Prüfgegenstands 10 entspricht somit dem Radius der Bohrung 12. Bei dieser konkreten Ausführungsform handelt es sich bei dem Prüfgegenstand 10 um eine Turbinenscheibe für eine Gas- oder Dampfturbine. FIG. 1 shows a schematic sectional view of a test object 10. The test object 10 is cylindrical. The test object 10 has a bore 12, which is aligned concentrically to the test object 10. The bore 12 and the test object 10 thus have a common rotational symmetry axis 14, which in FIG. 1 extends perpendicular to the plane of the drawing. The test object 10 has an outer radius r a and an inner radius r i . The inner radius r i of the test object 10 thus corresponds to the radius of the bore 12. In this specific embodiment, the test object 10 is a turbine disk for a gas or steam turbine.

An der Mantelfläche des Prüfgegenstands 10 befindet sich ein Prüfkopf 16. Der Prüfkopf 16 umfasst einen Ultraschallsender und einen Ultraschalldetektor. In dem Prüfgegenstand 10 sind weiterhin ein tangentialer Materialfehler 18 und ein radialer Materialfehler 20 dargestellt. Die Materialfehler 18 und 20 bilden jeweils einen Hohlraum im Prüfgegenstand 10. Der tangentiale Materialfehler 18 erstreckt sich bezüglich des zylindrischen Prüfgegenstands 10 im Wesentlichen in tangentialer Richtung. Entsprechend erstreckt sich der radiale Materialfehler 20 im Wesentlichen in radialer Richtung bezügliche des Prüfgegenstands 10.On the lateral surface of the test object 10 is a test head 16. The test head 16 includes an ultrasonic transmitter and an ultrasound detector. In the test article 10, a tangential material defect 18 and a radial material defect 20 are also shown. The material defects 18 and 20 each form a cavity in the test article 10. The tangential material defect 18 extends with respect to the cylindrical test article 10 substantially in the tangential direction. Accordingly, the radial material defect 20 extends substantially in the radial direction with respect to the test article 10.

Die Materialprüfung erfolgt, indem der Prüfkopf 16 an der äußeren Oberfläche des Prüfgegenstands 10 bewegt wird. FIG. 1 verdeutlicht, dass eine radiale Schallwelle 22 an dem tangentialen Materialfehler 18 besonders stark reflektiert wird, da der tangentiale Materialfehler 18 im Wesentlichen parallel zur Oberfläche des Prüfgegenstands 10 ausgerichtet ist. Ebenso wird deutlich, dass eine tangentiale Schallwelle 24 an dem radialen Materialfehler 18 besonders intensiv reflektiert wird.The material testing is done by moving the test head 16 on the outer surface of the test article 10. FIG. 1 illustrates that a radial sound wave 22 is particularly strongly reflected on the tangential material defect 18 because the tangential material defect 18 is oriented substantially parallel to the surface of the test article 10. It also becomes clear that a tangential sound wave 24 is reflected particularly intensely on the radial material defect 18.

Umgekehrt wird deutlich, dass die tangentiale Schallwelle 24 nur sehr schwach an dem tangentialen Materialfehler 18 reflektiert werden würde. Auch die radiale Schallwelle 22 würde nur wenig an dem radialen Materialfehler 20 reflektiert werden.Conversely, it becomes clear that the tangential sound wave 24 would only be reflected very weakly on the tangential material defect 18. Also, the radial sound wave 22 would be little reflected on the radial material defect 20.

Bei dem erfindungsgemäßen Verfahren erfolgt die Einschallung des Signals aus dem Prüfkopf 16 unter verschiedenen Winkeln. Dabei ist entweder der Prüfkopf 16 selbst oder zumindest dessen Schall emittierende Komponente derart schwenkbar, dass durch das Abtasten der äußeren Umfangsfläche das gesamte Volumen des Prüfgegenstands 10 zugänglich ist. Es werden dadurch insbesondere solche Materialfehler 20 leichter erfasst, deren Ausdehnung parallel zur Oberfläche des Prüfgegenstands 10 relativ gering sind. Dies wird bei dem zylinderförmigen Prüfgegenstand 10 beispielsweise dadurch erreicht, dass die Einschallrichtung neben der radialen Komponente auch eine tangentiale Komponente aufweist. Auch eine Einschallrichtung mit einer radialen und einer axialen wäre möglich. Schließlich kann sich die Einschallrichtung auch aus einer radialen, tangentialen und axialen Komponente zusammensetzen.In the method according to the invention, the sonication of the signal from the test head 16 takes place at different angles. In this case, either the test head 16 itself or at least its sound-emitting component is pivotable such that the entire volume of the test object 10 is accessible by scanning the outer peripheral surface. In particular, material defects 20 whose extent parallel to the surface of the test object 10 are relatively small are thereby more easily detected. This is achieved in the case of the cylindrical test object 10, for example, by virtue of the fact that the insonification direction has a tangential component in addition to the radial component. Also An insonification with a radial and an axial would be possible. Finally, the insonification can also be composed of a radial, tangential and axial component.

Beim Verfahren gemäß der Erfindung ist es nicht unbedingt notwendig, dass die gesamte Oberfläche oder der gesamte Oberflächenabschnitt abgetastet wird, um das gesamte Volumen des Prüfgegenstands 10 zu erfassen. Es kann beispielsweise eine bestimmte Strecke oder ein bestimmter Weg auf der Oberfläche abgetastet werden, da durch das Schwenken des Prüfkopfs 16 zumindest der relevante Bereich des Volumens auch ohne die vollständige Abtastung der Oberfläche erfasst werden kann.In the method according to the invention, it is not absolutely necessary that the entire surface or the entire surface portion is scanned in order to detect the entire volume of the test object 10. For example, it is possible to scan a certain distance or a specific path on the surface, since by pivoting the test head 16 at least the relevant area of the volume can be detected even without the complete scanning of the surface.

In FIG. 2 ist eine schematische Schnittansicht von oben auf den Prüfgegenstand 10 und den Prüfkopf 16 gemäß der Ausführungsform in FIG. 1 dargestellt. FIG. 2 zeigt die Bohrung 12, die Rotationssymmetrieachse 14 und die radiale Schallwelle 22. Der axiale Materialfehler 26 weist zumindest in Axialrichtung eine hinreichend große Ausdehnung auf. FIG. 2 verdeutlicht, dass die radiale Schallwelle 22 von dem axialen Materialfehler 26 hinreichend stark reflektiert wird. Auch die tangentiale Schallwelle 24 würde von dem axialen Materialfehler 26 bei einem nicht zu großen Einschallwinkel hinreichend stark reflektiert werden.In FIG. 2 is a schematic sectional view from above of the test object 10 and the test head 16 according to the embodiment in FIG. 1 shown. FIG. 2 shows the bore 12, the rotational symmetry axis 14 and the radial sound wave 22. The axial material defect 26 has a sufficiently large extent at least in the axial direction. FIG. 2 illustrates that the radial sound wave 22 is reflected by the axial material error 26 sufficiently strong. The tangential sound wave 24 would also be sufficiently strongly reflected by the axial material defect 26 at a not too large insonification angle.

FIG. 3 zeigt eine schematische Skizze über die geometrischen Beziehungen des Prüfgegenstands 10, des Prüfkopfs 16 und eines Materialfehlers 28 bei der bevorzugten Ausführungsform des erfindungsgemäßen Verfahrens. Zwischen dem Materialfehler 28 und der Rotationssymmetrieachse 14 ist ein radialer Abstand rs definiert. Der Schallweg s von dem Prüfkopf 16 bis zu dem Materialfehler 28 ist gegeben durch: s = r a 2 - r i 2 .

Figure imgb0001
FIG. 3 shows a schematic sketch of the geometric relationships of the test article 10, the test head 16 and a material error 28 in the preferred embodiment of the method according to the invention. Between the material error 28 and the rotational symmetry axis 14, a radial distance r s is defined. The sound path s from the probe 16 to the material defect 28 is given by: s = r a 2 - r i 2 ,
Figure imgb0001

Der Winkel zwischen dem Schallweg s und der Flächennormale ra bildet den Einschallwinkel α bzw. die Einschallrichtung. Der Schallweg s und der entsprechende Abstandsvektor rs des Materialfehlers 28 bilden einen rechten Winkel β.The angle between the sound path s and the surface normal r a forms the insonification angle α or the insonification direction. The sound path s and the corresponding distance vector r s of the material error 28 form a right angle β.

Durch die Verwendung des fokussierenden Prüfkopfs 16 wird der Schalldruck in der Nähe des Materialfehlers 28 erhöht. Dadurch wird das Signal-Rausch-Verhältnis verbessert. Dies ist jedoch nur innerhalb des Nahfelds sinnvoll. Die Länge n des Nahfelds ist gegeben durch: n = d 2 / 4 λ .

Figure imgb0002
By using the focusing probe 16, the sound pressure in the vicinity of the material defect 28 is increased. This improves the signal-to-noise ratio. However, this only makes sense within the near field. The length n of the near field is given by: n = d 2 / 4 λ ,
Figure imgb0002

Dabei ist d die Breite des Prüfkopfs 16 und λ ist die Wellenlänge der Schallwelle. Bei einer typischen Wellenlänge von λ = 5 mm und einer gewünschten Länge des Nahfelds von n = 1 m ist ein Prüfkopf 16 mit einer Breite von d = 140 mm erforderlich. Mit dem SAFT-Verfahren lässt sich diese Nahfeldlänge n auch ohne diese Breite erreichen. Bei dem SAFT-Verfahren wird ein breiter Prüfkopf simuliert und somit eine virtuelle Fokussierung erreicht.Here d is the width of the probe 16 and λ is the wavelength of the sound wave. At a typical wavelength of λ = 5 mm and a desired near-field length of n = 1 m, a test head 16 with a width of d = 140 mm is required. With the SAFT method, this near-field length n can also be achieved without this width. The SAFT method simulates a wide probe and thus achieves virtual focusing.

Die Amplitude der reflektierten Schallwelle hängt einerseits von der räumlichen Ausdehnung des Materialfehlers 28 und andererseits von den Reflexionseigenschaften an der Grenzfläche des Materialfehlers 28 ab.The amplitude of the reflected sound wave depends on the one hand on the spatial extent of the material defect 28 and on the other hand on the reflection properties at the interface of the material defect 28.

Es treten bei der Ultraschallmessung typischerweise zwei Arten von Rauschsignalen auf. Beim ersten Rauschsignal handelt es sich um jenes Rauschen, das bei jedem elektronischen Erfassungssystem, insbesondere in den Verstärkern auftritt. Dies lässt sich durch Mittelung reduzieren. Zwischen dem ersten Rauschsignal und dem reflektierten Schallsignal besteht keine Korrelation, insbesondere keine Phasenkorrelation. Das Aufsummieren der Signale führt daher zu einer Mittelung der Rauschsignale. Mit einer zunehmenden Anzahl von Summanden geht die Summe dieser Rauschsignale gegen Null, wenn die Rauschsignale keinen Gleichspannungsanteil enthalten. In der Praxis tritt entweder kein oder nur ein geringer Gleichspannungsanteil auf.There are typically two types of noise signals in the ultrasound measurement. The first noise signal is the noise that occurs in any electronic detection system, especially in the amplifiers. This can be reduced by averaging. There is no correlation between the first noise signal and the reflected sound signal, in particular no phase correlation. The summation of the signals therefore leads to an averaging of the noise signals. With an increasing number of summands the sum of these noise signals goes to zero, if the noise signals no DC voltage component included. In practice, either no or only a small DC component occurs.

Das zweite Rauschsignal kommt vom Prüfgegenstand selbst. Die Reflexionen an der Gefügestruktur des Metalls bilden einen Rauschteppich, der mit dem reflektierten Schallsignal korreliert. Bei dem Rauschteppich handelt es sich ebenfalls um ein reflektiertes Schallsignal. Es entsteht aus Reflexionen in polykristallinen Materialien an deren Korngrenzen und in Bereichen unterschiedlicher Orientierung der Kristallachsen. Kristalle sind akustisch anisotrop, so dass sich an den Korngrenzen der Wellenwiderstand ändert. Praktisch betrifft dies alle metallenen Werkstoffe. Die einzelnen Reflexionen aufgrund der Gefügestruktur sind nicht störend, aber in ausgedehnten Bereichen des Prüfgegenstands 10 kommt auf diese Weise das Rauschsignal zustande.The second noise signal comes from the test object itself. The reflections on the microstructure of the metal form a noise carpet, which correlates with the reflected sound signal. The noise carpet is also a reflected sound signal. It arises from reflections in polycrystalline materials at their grain boundaries and in regions of different orientation of the crystal axes. Crystals are acoustically anisotropic, so that the wave resistance changes at the grain boundaries. In practice this affects all metal materials. The individual reflections on the basis of the microstructure are not disturbing, but in broad areas of the test object 10, the noise signal comes about in this way.

Die Reflexionen am Gefüge und an den Materialfehlern können durch das SAFT-Verfahren getrennt werden. Das Gefügerauschen zeigt eine räumliche Statistik. Die Reflexionen am Gefüge sind phasenkorreliert. Die Summation im SAFT-Algorithmus führt dennoch zu einer relativen Schwächung der Reflexionen am Gefüge, da die Korngrenzen schwächer reflektieren als die Materialfehler. Wenn durch eine zufällig phasenrichtige Überlagerung der Amplituden mehrerer Korngrenzen eine Amplitudensumme zustande kommt, dann ist deren Winkel noch stärker eingeengt. Mit zunehmendem Winkelintervall nehmen die Amplituden aufgrund von Materialfehlern stärker zu als die durch die Korngrenzen bewirkten Amplituden.The reflections on the structure and on the material defects can be separated by the SAFT process. The Gefügerauschen shows a spatial statistics. The reflections on the microstructure are phase-correlated. The summation in the SAFT algorithm nevertheless leads to a relative weakening of the reflections on the microstructure, since the grain boundaries reflect less than the material errors. If an amplitude sum is achieved by a coincidentally in-phase superposition of the amplitudes of several grain boundaries, then their angle is narrowed even more. As the angular interval increases, the amplitudes due to material defects increase more than the amplitudes caused by the grain boundaries.

Für das erfindungsgemäße Verfahren wird beispielsweise ein Prüfkopf 16 mit einem Durchmesser von d = 24 mm verwendet. Beim SAFT-Algorithmus gemäß der Erfindung wird das Schallfeld des Prüfkopfs 16 berücksichtigt. Im Gegensatz dazu wird beim bekannten SAFT-Algorithmus die Größe des Prüfkopfs 16 vernachlässigt.For example, a test head 16 with a diameter of d = 24 mm is used for the method according to the invention. In the SAFT algorithm according to the invention, the sound field of the test head 16 is taken into account. In contrast, in the known SAFT algorithm, the size of the probe 16 is neglected.

Das erfasste Signal entsteht insbesondere durch den reflektierten Anteil eines Ultraschallimpulses an sprunghaften räumlichen Änderungen des Wellenwiderstands im Prüfgegenstand 10. Diese Änderungen werden als Materialfehler interpretiert, wenn dort keine konstruktiv bedingten Materialgrenzen oder Materialübergänge vorhanden sind. Das erfasste Signal enthält nur Informationen über die Amplitude und die Laufzeit. Da die Schallgeschwindigkeit im Material des Prüfgegenstands 10 bekannt ist, lässt sich aus der Laufzeit auch der Abstand bestimmen. Für die Ortsbestimmung in lateraler Richtung kann die räumliche Verteilung des Schallfeldes und der Empfindlichkeit des Prüfkopfs 16 verwendet werden.The detected signal is produced in particular by the reflected portion of an ultrasonic pulse at sudden spatial changes in the characteristic impedance in the test object 10. These changes are interpreted as material defects if there are no design-related material boundaries or material transitions. The detected signal contains only information about the amplitude and the transit time. Since the speed of sound in the material of the test object 10 is known, the distance can also be determined from the transit time. For spatial determination in the lateral direction, the spatial distribution of the sound field and the sensitivity of the test head 16 can be used.

Die Signale mit der Amplitude und der Laufzeit, die entlang des Weges vom Prüfkopf 16 erfasst werden, werden bezüglich des Ortes im Prüfgegenstand 10 laufzeitrichtig aufaddiert. Durch diese örtlich korrekte Zuordnung an den richtigen Ort nimmt die Amplitudensumme der Signale, die von einem bestimmten Ort im Prüfgegenstand kommen, mit jedem hinzuaddierten Signal um dessen Amplitude zu. Jedoch hängen die Amplituden von der Position des Prüfkopfs 16 und somit von der relativen Position des Materialfehlers 28 innerhalb des Schallfeldes ab.The signals with the amplitude and the transit time, which are detected along the way by the test head 16, are added up with respect to the location in the test object 10 with the correct time. As a result of this locally correct assignment to the correct location, the amplitude sum of the signals coming from a specific location in the test object increases by its amplitude with each added signal. However, the amplitudes depend on the position of the probe 16 and thus on the relative position of the material defect 28 within the sound field.

Der Mittelwert der Amplitude eines Materialfehlers ohne Richtwirkung ist proportional zu seinem Reflexionsvermögen gewichtet mit einem Faktor k. Der Faktor k ist ein Wert für die mittlere Empfindlichkeit entlang des Weges des Materialfehlers 18 durch das Schallfeld des Prüfkopfes 16. Auf diese Weise kann die erfasste Amplitude sinnvoll bewertet werden.The mean value of the amplitude of a material defect without directivity is proportional to its reflectance weighted by a factor k. The factor k is a value for the average sensitivity along the path of the material error 18 through the sound field of the test head 16. In this way, the detected amplitude can be meaningfully evaluated.

Bei dem Verfahren gemäß der Erfindung werden nicht einzelne erfasste Amplituden als Funktion der Zeit ausgewertet, sondern die berechneten räumlichen Amplitudenverteilungen. Diese lassen sich durch das SAFT-Verfahren rekonstruieren. Die berechneten räumlichen Amplitudenverteilungen haben ein höheres Signal/Rausch-Verhältnis als die direkt erfassten Amplituden. Auf diese Weise können Materialfehler einfacher identifiziert werden.In the method according to the invention, not individual detected amplitudes are evaluated as a function of time, but the calculated spatial amplitude distributions. These can be reconstructed by the SAFT method. The calculated spatial amplitude distributions have a higher signal-to-noise ratio than the directly detected amplitudes. In this way, material defects can be identified more easily.

Das Verfahren gemäß der Erfindung ermöglicht die Erweiterung der Anwendung der Reflektorbewertung gemäß dem AVG-Verfahren bei kleinen Amplituden durch eine relative Verringerung des Rauschens, wie es auch bei der Verwendung breiter Prüfköpfe 16 möglich wäre. Dabei liegt die Annahme zugrunde, dass die kleine Amplitude auf die geringe Größe des Reflektors zurück zu führen ist. Deshalb hat auch die geringe Richtwirkung des Reflektors, die auf die Beugung zurückzuführen ist, nur einen zu vernachlässigenden Einfluss auf die erfasste Amplitude.The method according to the invention makes it possible to extend the application of the reflector evaluation according to the AVG method at small amplitudes by a relative reduction of the noise, as would be possible with the use of wide probes 16. This is based on the assumption that the small amplitude is due to the small size of the reflector. Therefore, the low directivity of the reflector, which is due to the diffraction, only a negligible effect on the detected amplitude.

Das erfindungsgemäße Verfahren ermöglicht insbesondere die Untersuchung großer Prüfgegenstände 10 mit entsprechend großen Schallwegen. Diese großen Schallwege bewirken die geringen Amplituden.In particular, the method according to the invention makes it possible to examine large test objects 10 with correspondingly large sound paths. These large sound paths cause the low amplitudes.

Das erfindungsgemäße Verfahren ist auf bekannte klassische Prüftechniken anwendbar, bei denen der Prüfgegenstand mechanisch abgetastet wird und der Ort bzw. die Bewegung des Prüfkopfs 16 zu jedem erfassten Amplituden-Laufzeit-Diagramm bekannt ist.The inventive method is applicable to known classical testing techniques in which the test object is mechanically scanned and the location or the movement of the probe 16 is known to each detected amplitude-transit time diagram.

Die Bewertung der Amplitude erfolgt, indem zunächst der Reflektor von dem Schallfeld abgetastet wird. Die Winkelabhängigkeit der Amplitude innerhalb des Schallfeldes ist bekannt. Es werden m Amplituden in einem definierten Winkelintervall Δγ um die akustische Achse summiert. Daraus ergibt sich ein eindeutiger Zusammenhang zwischen der Amplitudensumme HSum und der Größe eines Referenzreflektors, der die gleiche Amplitudensumme HSum erzeugen würde.The evaluation of the amplitude is carried out by first the reflector is scanned by the sound field. The angular dependence of the amplitude within the sound field is known. M amplitudes are summed in a defined angular interval Δγ about the acoustic axis. This results in a clear relationship between the sum of the amplitudes H Sum and the size of a reference reflector which would produce the same amplitude sum H Sum .

Die Amplitudensumme HSum ist gegeben durch: H Sum = Σ H i γ i ,

Figure imgb0003
wobei über die Anzahl m der erfassten Amplituden summiert wird. Dabei sind Hi die erfassten Amplituden bei den einzelnen Messungen und γi der Winkelabstand zur akustischen Achse. Bei einem festen Abstand der Messpunkte sind auch die Winkelabstände bei den einzelnen Messungen in etwa äquidistant. Mit zunehmender Anzahl m der Einzelmessungen nähert sich der Korrekturfaktor k einem Grenzwert, der der mittleren Empfindlichkeit in dem Winkelintervall Δγ entspricht. Der für das AVG-Verfahren relevante Abstand zwischen dem Materialfehler 18 und dem Prüfkopf 16 ergibt sich aus der Position des Prüfkopfs 16, wenn der bestimmte Ort des Materialfehlers 18 auf der akustischen Achse liegt.The amplitude sum H Sum is given by: H Sum = Σ H i γ i .
Figure imgb0003
wherein is summed over the number m of the detected amplitudes. H i are the detected amplitudes in the individual measurements and γ i the angular distance to the acoustic axis. At a fixed distance of the measuring points and the angular distances in the individual measurements are approximately equidistant. As the number m of the individual measurements increases, the correction factor k approaches a threshold corresponding to the average sensitivity in the angular interval Δγ. The distance between the material defect 18 and the test head 16 that is relevant for the AVG process results from the position of the test head 16 when the specific location of the material defect 18 lies on the acoustic axis.

Zwischen der Amplitudensumme HSum und der Amplitude HAVG gemäß dem AVG-Verfahren besteht die Beziehung: H AVG = H Sum / m * k ,

Figure imgb0004
wobei m die Anzahl der Einzelmessungen und k ein Korrekturfaktor ist. Der Korrekturfaktor k ist gegeben durch: k = 1 / m Σ H 0 γ i ,
Figure imgb0005
wobei über die Anzahl m der erfassten Amplituden summiert wird. Dabei ist H0i) die winkelabhängige Amplitudenverteilung im Schallfeld des Prüfkopfes 16, die auf H0(γ=0)=1 normiert ist.Between the sum of the amplitudes H Sum and the amplitude H AVG according to the AVG method, the relation exists: H AVG = H Sum / m * k .
Figure imgb0004
where m is the number of individual measurements and k is a correction factor. The correction factor k is given by: k = 1 / m Σ H 0 γ i .
Figure imgb0005
wherein is summed over the number m of the detected amplitudes. Here, H 0i ) is the angle-dependent amplitude distribution in the sound field of the test head 16, which is normalized to H 0 (γ = 0) = 1.

Mit zunehmender Größe des Materialfehlers, d.h. des Reflektors, nimmt auch dessen Richtwirkung zu. Dies kann bei größeren Materialfehlern und bei einer mittleren Schräglage zu einer Unterbewertung der Amplitude in dem Winkelintervall Δγ führen und sollte daher berücksichtigt werden. Insbesondere ist das Verfahren für kleinere Materialfehler geeignet, deren Richtwirkung von untergeordneter Bedeutung ist.With increasing size of the material error, ie the reflector, its directivity also increases. This may lead to an underestimation of the amplitude in the angular interval Δγ for larger material errors and at an average skew and should therefore be taken into account. In particular, the method is suitable for minor material defects whose directivity is of minor importance.

In FIG. 4 ist eine schematische Schnittansicht auf den Prüfgegenstand 10 und einen fokussierenden Prüfkopf 16 gemäß dem Stand der Technik dargestellt. Der Prüfgegenstand 10 weist einen Materialfehler 30 auf. An der Außenseite des Prüfgegenstands 10 befindet sich der Prüfkopf 16, der als fokussierender Prüfkopf ausgebildet ist. Von dem Prüfkopf 16 werden fokussierte Schallwellen 32, 34 und 36 abgestrahlt.In FIG. 4 is a schematic sectional view of the test object 10 and a focussing probe 16 according to the prior art shown. The test article 10 has a material defect 30. On the outside of the test object 10 is the test head 16, which is designed as a focusing probe. From the probe 16 focused sound waves 32, 34 and 36 are emitted.

Dabei stellt die durchgezogene Linie die Wellenfront der aktuellen Schallwelle 32 dar. Die gestrichelten Linien stellen die Wellenfronten der früheren Schallwellen 34 und der späteren Schallwellen 36 dar. Die fokussierten Schallwellen 32, 34 und 36 breiten sich entlang einer vorbestimmten Richtung mit seitlich begrenzter Ausdehnung aus. Die fokussierte Schallwellen 18 und 20 breiten sich somit nicht kugelförmig im gesamten Halbraum aus.The solid line represents the wavefront of the current sound wave 32. The dashed lines represent the wavefronts of the earlier sound waves 34 and the later sound waves 36. The focused sound waves 32, 34 and 36 propagate along a predetermined direction with laterally limited extension. The focused sound waves 18 and 20 thus do not propagate globally in the entire half space.

Der Prüfkopf 16 bewegt sich während des Abtastens auf der Oberfläche des Prüfgegenstands 10 entlang einer Abtastrichtung 38. Die Fokussierung tritt jedoch nur innerhalb des Nahfelds des Prüfkopfs 16 auf. Je größer die Breite des Prüfkopfs 16 senkrecht zur Abstrahlrichtung ist, desto größer ist die Länge des Nahfelds und somit die Eindringtiefe der fokussierten Schallwellen 32, 34 und 36.The probe 16 moves along the scan 10 surface along a scan direction 38 during scanning. However, focus only occurs within the near field of the probe 16. The greater the width of the test head 16 perpendicular to the emission direction, the greater the length of the near field and thus the penetration depth of the focused sound waves 32, 34 and 36.

FIG. 5 zeigt eine schematische Schnittansicht auf den Prüfgegenstand 10 und den Prüfkopf 16 nach dem SAFT-Verfahren gemäß dem Stand der Technik. Der Prüfgegenstand 10 ist mit dem Materialfehler 30 dargestellt. An der Außenseite des Prüfgegenstands 10 befindet sich der Prüfkopf 16. Der Prüfkopf 16 hat im Vergleich zu FIG. 4 einen relativ kleinen Durchmesser und ist nicht fokussierend ausgebildet. FIG. 5 shows a schematic sectional view of the test object 10 and the test head 16 according to the SAFT method according to the prior art. The test object 10 is shown with the material error 30. On the outside of the test object 10 is the test head 16. The test head 16 has compared to FIG. 4 a relatively small diameter and is not focused.

Von dem Prüfkopf 16 werden kugelschalenförmige Schallwellen 42, 44 und 46 abgestrahlt. Die Wellenfront der aktuellen kugelschalenförmigen Schallwelle 42 ist durch eine durchgezogene Linie dargestellt. Die gestrichelten Linien stellen die Wellenfronten der früheren kugelschalenförmigen Schallwellen 44 sowie der späteren kugelschalenförmigen Schallwellen 46 dar. Ein Vergleich von FIG. 4 und FIG. 5 verdeutlicht, dass die Wellenfronten 32, 34 und 36 einerseits und 42, 44 und 46 andererseits entgegengesetzt gekrümmt sind.From the test head 16 spherical shell-shaped sound waves 42, 44 and 46 are emitted. The wavefront of the current spherical shell-shaped sound wave 42 is represented by a solid line. The dashed lines represent the wavefronts of the former spherical shell-shaped Sound waves 44 and the later spherical shell-shaped sound waves 46. A comparison of FIG. 4 and FIG. 5 illustrates that the wavefronts 32, 34 and 36 on the one hand and 42, 44 and 46 on the other hand are oppositely curved.

Der Prüfgegenstand 10 wird bei diesem SAFT-Verfahren von einem Rechner in Volumenelemente unterteilt. Jedes Volumenelement wird während der Abtastung nacheinander als Reflektor betrachtet. Die reflektierten Signalanteile von verschiedenen Positionen des Prüfkopfs 16, die zu demselben Volumenelement gehören, werden aufgezeichnet und mit Unterstützung des Rechners phasenrichtig aufaddiert. Auf diese Weise werden nur für solche Orte mit tatsächlicher Reflexion aufgrund der konstruktiven Interferenz Echosignale mit großer Amplitude erhalten.The test object 10 is subdivided by a computer into volume elements in this SAFT method. Each volume element is considered as a reflector during the scan. The reflected signal components from different positions of the probe 16, which belong to the same volume element, are recorded and added in phase with the assistance of the computer. In this way, echo signals of large amplitude are obtained only for those places with actual reflection due to constructive interference.

Für Orte ohne tatsächliche Reflexion werden aufgrund der destruktiven Interferenz die Echosignale ausgelöscht. Der Abtast- und Rechenvorgang simuliert bei konstruktiver Interferenz einen Ultraschalldetektor, dessen Größe der abgetasteten Fläche entspricht. Bei diesem bekannten SAFT-Verfahren beträgt der Einschallwinkel stets 0° und die gesamte Oberfläche des Prüfgegenstands 10 wird abgetastet.For locations without actual reflection, the echo signals are canceled due to destructive interference. The scanning and arithmetic operation simulates constructive interference an ultrasonic detector whose size corresponds to the scanned area. In this known SAFT method, the insonification angle is always 0 ° and the entire surface of the test object 10 is scanned.

Gemäß der Erfindung ist im Gegensatz dazu der Einschallwinkel α variierbar.According to the invention, in contrast, the insonification angle α can be varied.

Das erfindungsgemäße Verfahren ist nicht auf den zylindrischen Prüfgegenstand 10, wie Radscheiben oder Wellen, beschränkt. Die Einschallrichtung kann aus geeigneten Basisvektoren zusammengesetzt werden, die an die geometrische Form des Prüfgegenstands 10 angepasst sind.The method according to the invention is not limited to the cylindrical test object 10, such as wheel disks or shafts. The insonification direction can be composed of suitable base vectors, which are adapted to the geometric shape of the test object 10.

Weiterhin kann es bei einer geeigneten Wahl der Schwenkachsen des Prüfkopfs 16 ausreichen, dass nicht eine ganze Oberfläche, sondern nur entlang einer vorbestimmten Strecke oder eines vorbestimmten Weges abgetastet werden muss. Das erfindungsgemäße Verfahren eröffnet somit mehrere Möglichkeiten, um das gesamte Volumen des Prüfgegenstands 10 hinreichend zu erfassen.Furthermore, with a suitable choice of the pivot axes of the test head 16, it may be sufficient that it is not necessary to scan an entire surface, but only along a predetermined path or a predetermined path. The inventive method thus opens up several Possibilities to adequately capture the entire volume of the test article 10.

Das erfindungsgemäße Verfahren führt zu einer wesentlichen Verbesserung der Erkennbarkeit kleiner Materialfehler und solcher, die sich tief in Inneren des Prüfgegenstands 10 befinden.The method according to the invention leads to a substantial improvement in the detectability of small material defects and those which are located deep inside the test object 10.

Claims (17)

  1. Method for nondestructive material testing of an at least sectionally solid test object (10) by the application of ultrasonic waves (20, 24) to the test object (10) and the detection of the ultrasonic waves reflected inside the test object (10), the method having the following steps:
    a) computer-aided subdivision of the test object (10) into a predetermined number of volume elements,
    b) application of ultrasound to the test object (10) at a multiplicity of surface elements while scanning the surface of at least one surface section of the test object (10),
    c) detection of the sound waves reflected at the volume elements while scanning the multiplicity of surface elements on the surface or at least on the surface section of the test object (10), and
    d) in-phase addition of the sound waves reflected at the same volume elements and detected at various surface elements of the surface of the test object (10),
    characterized in that
    e) consideration is given to an angle-dependent amplitude distribution (H0) in the sound field of a test head (16), in order to consider different sensitivities that depend on the angle,
    - the angle-dependent amplitude distribution (H0) in the sound field of the test head (16) is used to determine the amplitudes of the reflected sound waves
    - in step d) the amplitudes of the sound waves are added up in phase within a predetermined angular interval (Δγ) about the acoustic axis of the sound field of the test head (16), and
    - a specific number (m) of amplitudes are summed within the predetermined angular interval (Δγ) about the acoustic axis, in order to calculate the size of a reference reflector that would produce the same amplitude sum (Hsum).
  2. Method according to Claim 1, characterized in that the angle-dependent amplitude distribution (H0) is used to determine a correction factor (k) that corresponds to the mean sensitivity along the path through the sound field of the test head (16).
  3. Method according to either of the preceding claims, characterized in that the application of ultrasound to the test object (10) is performed at various insonification angles (α) with reference to the surface element on the surface of the test object (10).
  4. Method according to Claim 3, characterized in that the insonification angles (α) lie within a cone whose axis of symmetry forms the normal to the respective surface element.
  5. Method according to one of the preceding claims, characterized in that the surface or at least the surface section of the test object (10) is scanned along a predetermined line.
  6. Method according to one of the preceding claims, characterized in that the surface or at least the surface section of the test object (10) is scanned in accordance with a predetermined scheme.
  7. Method according to one of the preceding claims, characterized in that the surface or at least the surface section of the test object (10) is completely scanned.
  8. Method according to one of the preceding claims and one of Claims 3 and 4, characterized in that the insonification angles (α) are between 0° and 50°, preferably between 0° and 30°.
  9. Method according to one of the preceding claims, characterized in that the method is provided for an at least sectionally rotationally symmetrical test object (10).
  10. Method according to one of the preceding claims, characterized in that the method is provided for an at least sectionally cylindrical test object (10).
  11. Method according to Claim 10, characterized in that the insonification direction has a radial, tangential and/or axial component with reference to the surface of the cylindrical test object (10).
  12. Method according to one of the preceding claims, characterized in that the method is provided for the material testing of a test object (10) made from metal.
  13. Method according to one of the preceding claims, characterized in that the method is provided for the material testing of a forged component (10).
  14. Method according to one of the preceding claims, characterized in that the method is provided for the material testing of a turbine wheel.
  15. Device for nondestructive material testing of an at least sectionally solid test object (10), which device has at least one test head (16) for the emission of ultrasonic waves (20, 24) and for the detection of the ultrasonic waves reflected inside the test object (10),
    characterized in that
    a computer is present which is set up to carry out the steps of the method according to at least one of Claims 1 to 14.
  16. Device according to Claim 15, characterized in that the test head (16) or at least its component emitting sound is swivel mounted such that the insonification direction can be varied with reference to the surface normal to the surface of the test object (10).
  17. Device according to Claim 15 or Claim 16, characterized in that the test head (16) is mounted to swivel between 0° and 60°, preferably between 0° and 30°, with reference to the surface normal to the surface of the test object (10).
EP08735834A 2007-05-15 2008-04-04 Method and device for non-destructive material testing of a test object using ultrasonic waves Not-in-force EP2147300B8 (en)

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KR101197323B1 (en) 2012-11-05
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US8656782B2 (en) 2014-02-25
EP2147300B8 (en) 2013-01-23
CN101711358B (en) 2013-07-24
WO2008138684A1 (en) 2008-11-20
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EP2147300A1 (en) 2010-01-27
CN101711358A (en) 2010-05-19

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